Image Credit: NASA/JPL-Caltech / University of Arizona
It is rare in astronomy to work on objects that change. Most stars will look the same in ten years as they do now, and will be in the same place. The surface of the moon changes so slowly that Apollo astronaut footprints probably still look as if they were made yesterday, not 40 years ago. Some items do change in short timescales -- supernova explosions appear and disappear on timescales of weeks or months, comets come and go, the clouds of Jupiter are constantly changing.
Mars is one of those objects that tends to change slowly. The polar ice caps come and go in a Martian year (about two Earth-years), and dust storms come and go in the Martian summer. But, otherwise, the surface changes on timescales of millions of years. Sometimes, however, we are surprised.
The pictures above were taken by the Mars Reconnaissance Orbiter (MRO), a robotic space probe in orbit around Mars. The pictures are part of a study to observe how Mars's polar ice caps change through the seasons. Mars has ice caps, like the Earth, only Mars's ice caps contain both water ice and carbon dioxide ice ("dry ice"). When this picture was taken, it was spring on the north pole, so scientists wanted to see what happened as the ice caps melted (more precisely, they sublimate, turning straight from ice into gas).
What you see in the picture is the carbon dioxide ice (brighter white) on top of some dusty water ice (the light red colors) and what is probably a permafrost mix of ice and rock (dark colors). The edges are a cliff over 2000 feet high. And, in the middle, you see the dust from an avalanche that happened just as the picture was taken.
First and foremost, I find the picture cool. Who would've thought we'd ever take pictures of avalanches on Mars?
But there is some science we can learn, too. Since these pictures were taken at random, and not as part of a long time series, we know that the avalanches must be common. Why? Because if the avalanches were rare, the chances of seeing one in a single picture would be very small. The chances of seeing four in separate pictures would be even smaller. Imagine you are driving across the country, taking pictures every few minutes. You wouldn't be surprised that lots of your pictures have telephone poles, because telephone poles are very common. But what are the chances that one of your pictures would contain a penguin? Very small -- the number of loose penguins in the Northern Hemisphere is tiny, if not zero. So, suppose you were driving past Chicago and four of your pictures had penguins. You would instantly know that there must be a lot of penguins in Chicago -- your next step would be to figure out why.
And that is the next step for the Mars avalanche researchers. Why are avalanches so common? Do those avalanches greatly change the rocky landscape, or are they mostly ice, and so they evaporate in the Martian spring and leave no trace behind?